ASTRONOMERS GLIMPSE TENUOUS GAS FILLING THE EARLY UNIVERSE

     Astronomers are announcing today (January 5, 1999) that they have 
detected and measured the tenuous gas that filled the Universe about two 
billion years after its creation in the Big Bang. The report is being 
presented by Dr. Sara R. Heap of NASA's Goddard Space Flight Center, 
Greenbelt MD and colleagues at Goddard and Princeton University Observatory 
in Princeton, NJ to the American Astronomical Society meeting in Austin, Texas.
The result is noteworthy because this tenuous gas is the primordial stuff out 
of which galaxies are made, and understanding the properties of this gas 
gives important clues about how galaxies are formed.   

"This observation tells us about the mud from which the bricks are made
to assemble the galactic houses," said Dr. Gerard Williger, also at 
Goddard Space Flight Canter. "The Hubble Deep Field images show houses 
and bricks (galaxies and proto-galaxies); we see what is happening to the 
mud." 

To detect this tenuous gas, the scientists used the Space Telescope Imaging 
Spectrograph (STIS) on-board the Hubble Space Telescope (HST) to record the 
spectrum of a distant quasi-stellar object (QSO) designated Q 0302-003 in the 
constellation Cetus (the whale). This QSO has a redshift of 3.286, which 
means that extreme-ultraviolet light, which acts as a tracer of the tenuous 
gas, is redshifted into ultraviolet wavelengths where it can be observed 
by STIS. It also means that the light we receive from this quasar was emitted 
when the Universe was 4.286 times smaller than it is now.

A spectrograph measures the brightness of light from an object at different 
colors, which correspond to different wavelengths. Red light has a longer 
wavelength and less energy than blue light, which has a longer wavelength 
than the more powerful ultraviolet light, which is invisible to the human 
eye. This variation of brightness with wavelength is called the spectrum 
of the object.

The tenuous gas between the galaxies, also called the intergalactic
medium, is composed primarily of hydrogen and helium. Hydrogen does not 
work as a tracer of the intergalactic medium because practically all 
hydrogen atoms have lost their electrons and are therefore incapable of 
absorbing light. Helium atoms in the intergalactic gas are able to absorb 
extreme-ultraviolet light, effectively blocking the QSO from view. However, 
given sufficient energy, helium atoms also lose both their surrounding 
electrons - turning into a state called fully ionized - which prevents them 
from absorbing light. Regions with fully ionized helium are therefore 
transparent to ultraviolet light.

The scientists believe the new observations promise to put current
theories of the evolution of the Universe on a more realistic footing.
The STIS spectrum indicates that the tenuous gas between the galaxies stayed 
opaque to extreme-ultraviolet light until a later epoch than previously 
thought. This means that the whole Universe was not bathed in extreme-
ultraviolet light as it is now. Instead, light from energetic objects such 
as quasars only affects small regions around these objects.

The STIS observation reveals a bright area in the spectrum of the QSO, 
which the scientists interpret as the signature of a region that has been 
made transparent by an ionizing source lying along the line of sight to 
the QSO. The source is likely to be a remote, young galaxy, possibly with 
a central black hole as its main power source.

The STIS spectrum also shows that the intergalactic helium gas closer to 
Earth (and more recent in time) is fully ionized and transparent to extreme 
ultraviolet light. The researchers suggest that before that time, there were 
not enough galaxies yet formed in the Universe to fully ionize all the helium. 
The transition from opaque to transparent may mark the era in the evolution 
of the Universe when a lot of galaxies formed and "turned on" like lightbulbs.
"We were very surprised to find that the transition from opaque to transparent
was quite abrupt. It indicates a relatively rapid increase in the ionization
state of the intergalactic medium." said Heap.

A paper reporting these findings was submitted to the Astrophysical Journal 
on 23 December 1998. The authors are: Sara R. Heap, Gerard M. Williger, 
Alain Smette, Ivan Hubeny, Meena Sahu -- all from NASA's Goddard Space Flight 
Center -- and Edward B. Jenkins and Todd M. Tripp of Princeton University 
Observatory, and summer student Jonathan N. Winkler, now at Oxford University, 
England.


For further information contact: 
Sara R. Heap 
301-286-5359
heap@srh.gsfc.nasa.gov

Gerard M. Williger
301-286-5906
williger@tejut.gsfc.nasa.gov